Generally, a vehicular automatically shiftable transmission includes a number of gear elements coupling its input and output shafts, and a number of torque establishing devices, such as clutches and brakes, which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts. The brake can be of the band type or disk type; those skilled in the automotive art refer to disc type brakes in transmissions as “clutches” or “reaction clutches”. As used herein, the terms “clutches” and “torque transmitting devices” will be used to refer to brakes as well as clutches.
The input shaft is typically connected to the vehicle's engine through a fluid coupling, such as a torque converter, and the output shaft is operatively connected to the vehicle's wheels. The shifting from one forward speed ratio to another is undertaken in response to engine load, such as throttle opening, and vehicle speed. This shifting generally involves releasing or disengaging a clutch (off-going) associated with the current speed ratio and applying or engaging a clutch (on-coming) associated with the desired speed ratio. In this disclosure, a double transition is featured, wherein two clutches associated with the current speed ratio are released and two other clutches associated with the desired speed ratio are engaged.
The speed ratio is defined as the transmission input speed or turbine speed divided by the output speed; thus, a low gear range has a high speed ratio and a higher rear range has a lower speed ratio. To perform an up-shift, a shift is made from a high speed ratio to a low speed ratio. In the type of transmission involved in this invention, the up-shift from low range to first range is accomplished by disengaging two clutches associated with the higher speed ratio and engaging two clutches associated with the lower, or target, speed ratio to thereby reconfigure the gear set to operate at the lower speed ratio. Shifting performed in the above manner require precise timing in order to achieve high quality shift feel. In the case of double transition shifts, particular care must be taken to avoid clutch tie-up or a momentary shift to neutral during the range shift, and instead to provide power throughout the shift.